FIG. 10 shows conventional optical reflection element 1. Optical reflection element 1 includes mirror portion 2, a pair of first oscillators 3 and 4, frame body 5, and a pair of second oscillators 6 and 7. First oscillators 3 and 4 are coupled to the ends of mirror portion 2. Frame body 5 is coupled to first oscillators 3 and 4, and surrounds the outer peripheries of first oscillators 3, 4 and mirror portion 2. Second oscillators 6 and 7 are coupled to the ends of frame body 5.
First oscillators 3 and 4 have an axis S1 as their central axis, which is parallel to the y-axis. Second oscillators 6 and 7 have an axis S2 as their central axis, which is parallel to the x-axis. Thus, first oscillators 3, 4 and second oscillators 6, 7 are formed in a meandering shape.
First oscillators 3, 4 and second oscillators 6, 7 include a drive element. The drive element is composed of a lower electrode layer, a piezoelectric layer, and an upper electrode layer. By applying voltage to the drive element, mirror portion 2 rotates about axes S1 and S2. Then, by applying light to mirror portion 2 while it is rotating, the x-y surface of a screen can be scanned with reflected light. As a result, an image can be projected onto a wall, a screen, or the like.
First oscillators 3, 4, second oscillators 6, 7, and mirror portion 2 also include a monitor element. The monitor element is also composed of a lower electrode layer, a piezoelectric layer, and an upper electrode layer. The monitor element detects an electrical signal, and supplies the signal to the upper electrode layer of the drive element via a feedback circuit. As a result, in theory, optical reflection element 1 can be driven constantly at the resonant frequency, thereby having a large amplitude. Such optical reflection elements are referred to as self-excited driving type.
An example of a conventional technique related to the present invention is shown in Patent Literature 1.
In this example, however, when the driving frequency is too high, the optical reflection element sometimes cannot perform self-excited driving.
The reason for this is considered as follows. Arranging a ground electrode lengthwise increases the resistance. The increased resistance causes current leakage between the upper electrode layer of the monitor element and the upper electrode layer of the drive element adjacent to the monitor element.
This decreases the detection accuracy of the monitor element, making is impossible for the optical reflection element to perform self-excited driving.